1. Field of the Invention
The invention relates to motorcycle frames formed by casting and to motorcycles provided with such frames.
2. Description of Background Art
It is known that a frame for motorcycles may be cast and formed integrally from a light alloy such as aluminum or the like. It is also known to provide a pair of upper and lower ribs on side surfaces of a vehicle body frame. (See Japanese patent laid open No. 61-6084). Using such a process, it is possible to select the thickness of the frame which can thus be made to correspond to load distribution. Accordingly, appropriate distribution of strength can be attained and also appropriate resilience can be obtained by forming a relatively thin wall extending over a wide area. Moreover, frame rigidity may be improved by the side ribs whereby it is possible to reduce weight.
From Japanese patent laid open No. 61-215187, it is known to provide a motorcycle frame cast from aluminum and having a head pipe portion, a main frame portion and a mounting portion for a rear swing arm, wherein a side access port for a tool is formed in the main frame portion to provide access to a carburetor.
It is known from Japanese patent laid open No. 61-202983 to provide a motorcycle frame having a pair of left and right tubular frame members of rectangular cross section and formed of aluminum alloy. An air flow passage is defined by the frame members and a bottom surface of a fuel tank resting on the frame members, and a carburetor is arranged in the air flow passage to receive cooling air. In this arrangement the engine must be spaced sufficiently from the fuel tank to prevent excessive engine heat being transmitted to the fuel tank. Further, the two frame members project laterally from the sides of the engine beyond its width leading to a large overall transverse width.
Japanese patent laid open No. 61-215186 discloses a frame case from aluminum alloy with a pair of left and right frame portions, wherein a cross pipe is fitted between cavities formed in rear inwardly facing surfaces of the frame portions. On a bracket provided in a middle portion of the cross pipe is mounted an upper end of a rear cushion unit of a rear suspension. To a rear portion of the main frame a separate bracket is secured to act as a rear swing arm supporting portion. In this arrangement, the use of laterally spaced frame portions and the separate cross pipe leads to relatively large stress concentrations caused by loads from the rear cushion unit. The significant length of the cross pipe is disadvantageous in withstanding such loads. Further, since there are several separate parts, such as the bracket for supporting the rear swing arm, a bracket provided on the cross pipe and so forth, there are many steps on an assembly line.
Finally, it is known that a rear swing arm may be cast in aluminum or the like, for example, using partially a sand mould. (See Japanese patent laid open No. 61-125189). In this proposal, an arm portion is formed in a generally inverted "U" shape in transverse section and a number of transverse ribs are integrally provided in the "U" shaped space.
Viewed from one aspect, the invention provides a motorcycle frame formed by casting and having a transverse section in the form of an inverted "U", the frame being cast in two parts which are connected together substantially along the longitudinal centerline of the motorcycle.
In such an arrangement, the space between the arms of the "U" may be advantageously used to locate the motorcycle engine and other parts, while the "U" shape of the frame ensures sufficient rigidity. The arms of the "U" will thus normally define two side walls and there will be an upper wall extending between the side walls. Assembly of a motorcycle is facilitated by forming the frame in two parts.
Preferably, the frame further comprises a head pipe supporting portion, an engine supporting portion, and a rear swing arm supporting portion, wherein the frame is reinforced by a plurality of ribs the density of distribution of which varies, such ribs being most densely distributed at least in the rear swing arm supporting portion. The weight of the frame may thus be kept low by providing densely distributed reinforcing ribs in the rear swing arm supporting portion where they are most needed to withstand loads imposed during use of the motorcycle, and less densely distributed ribs in other areas where stresses are likely to be less.
If the frame is an aluminum casting formed by means of a sand mould, the lowest limit of the thickness is on the order of 4 mm, and therefore, the frame is relatively heavy in its weight. On the other hand, with an aluminum die casting process, the forming performance and strength is balanced when the thickness is about 2.5 mm. When the thickness exceeds 4 mm, cavities may be formed so that it is difficult to increase the strength of the product simply by increasing the thickness. The rib arrangement discussed above is thus particularly advantageous if the frame is formed as an aluminum die casting, since the ribs stiffen the frame structure as necessary without increasing the wall thickness of the frame.